The present invention relates generally to integrated circuit devices and packaging methods, and, more particularly, to an apparatus and method for mechanical coupling of land grid array (LGA) applications.
Area array socket connectors are an evolving technology in which an electrical interconnection between mating surfaces is provided through a conductive interposer. One significant application of this technology is the socketing of land grid array (LGA) modules directly to a printed wiring board, in which the electrical connection is achieved by aligning the contact array of the two mating surfaces and the interposer, then mechanically compressing the interposer. LGA socket assemblies are prevalent today in the electronics industry, and are commonly used to attach single chip modules (SCMs) to printed wiring boards. However, the demand for higher performance is driving the requirement to develop LGA socket assemblies for multi-chip modules (MCMs).
One of the problems confronting the design of multi-chip module socket assemblies is developing a mechanical system to generate the large force required to compress the increased contact area of the interposer without interfering with the heat transfer system of the module, distorting the printed wiring board, or occupying a significant portion of the volume of the enclosure in which the module is housed. A system has thus been devised to solve this problem utilizing one or more load posts attached to the module. The load posts pass though the printed wiring board and a mechanical system incorporating a stiffening plate to reduce printed wiring board distortion is then employed to exert a large tensile force on the load posts pulling the module toward the printed wiring board and compressing the interposer. Since the mechanical system used to compress the interposer is on the side of the printed wiring board opposite the module, it may not occupy any volume of the enclosure nor interfere with heat transfer. One drawback of this system is that if the printed wiring board is mounted vertically, it is very difficult for one person to actuate the connection system on the opposite side of the printed wiring board while holding the multi-chip module in place.
Moreover, there is at present no existing mechanical coupling solution for an organic, LGA configured package. Generally, the interconnect chosen for organic packaging is of the column or ball grid array type, for example, since more is known about the static forces associated therewith. As indicated above, LGA packages create complex stresses on the carrier (via the chip) due to the force per I/O of the retention hardware. This force must be great enough to both compress the LGA springs adequately and to provide stable electrical contacts. Unfortunately, an organic carrier experiences strain and possible failed signal lines (i.e., an electrical reliability impact) under LGA loading. In addition, the stress created on the chip (depending on the stress field) can also negatively affect the low-k and ultra low-k materials and associated BEOL structure of the semiconductor device.
LGA contacts generally apply a force-per-contact to the substrate in a direction that is perpendicular (i.e., normal) to the X-Y plane of the carrier. This normal force may be in the range of about 65 to 85 grams per contact or more, depending on the type of contact used, the substrate co-planarity, and the actuation system tolerances. Attempts to lower this contact force have heretofore been limited by contact stability, reliability concerns and mass isolation effects, among others. LGA contacts tested to date have generally exhibited electrical stability around 28 to 30 grams of normal force per contact. Using a minimum contact normal force of 30 grams, and accounting for tolerance and mass isolation effects, has resulted the nominal force to be in the 65 to 85 gram range.
In view of the above, it would be desirable to be able to provide a low-mass method of securing and supporting an organic laminate top surface to a heat sink device, thereby completing the load application path from the heat sink in an LGA contact system.